Backache is one of the oldest and most ubiquitous complaints offered by patients to their physicians. After the common cold, it is the most common reason for Americans to seek medical attention, and it is the most common cause of disability not only in America but in most technologically developed countries. The costs associated with the overall medical and surgical treatment of back pain in the United States are currently estimated at a staggering $90 B annually, and this does not begin to measure the loss of wages and productivity, as well as disability benefits, legal costs, and other expenses borne by the system. Overall, the global cost of back pain in America is virtually immeasurable.
The earliest records of interactions between physicians and their patients, including records from the Egyptian artifacts such as the famous Edwin Smith Papyrus, discuss maladies affecting the spine. There are references to backaches seen in some of the earliest writings from the land of Sumer, as well as a wealth of literature discussing a variety of potions and methods of relief of back pain that can be found in the literature of the ancient Indian medicine, which dates as far back as four thousand years. Despite the broad centuries and advances in medical science, there is no clear, universally accepted explanation for back pain secondary to degenerative disease. This definition fully recognizes and excludes the many instances in which back pain has a very clearly defined pathologic basis (i.e. tumor, fracture, nephrolithiasis, infection, disorders of the internal organs such as penetrating duodenal ulcers and so forth).
The first scientific attempts at understanding back pain resulting from degenerative changes or “arthritis,” came soon after the introduction of the x-ray by Roentgen in 1897. With this new tool, the types of changes that were associated with degenerative changes were observed and quickly characterized in the endplates, disc spaces, and facet joints. Soon it was appreciated that if a patient with chronic back pain and severe degenerative changes was followed with serial x-rays, a subset of these patients would ultimately report improvement if autofusion was observed in one or more segments.
These observations led to the speculation that lower back pain associated with degenerative disease is the result of excessive or abnormal movement of the vertebrae as they related to one another, resulting from relative incompetence of the disc and facet joints as the degenerative process advances. Extrapolating from this, surgical fusion of the spine was first introduced as a treatment option for patients with degenerative disease of the spine in the early part of the 20th century. The early reports of some success with spine fusion further helped support these contentions.
However, this is obviously vastly over-simplifying both the biomechanic and global physiologic scheme of the spine. The spine is a superbly (probably divinely) engineered articulated column, which is designed, in its ideal embodiment, to subserve a number of functions. Most importantly, it serves as a protective encasement for the most delicate structure in nature, the human spinal cord, and the associated nerves comprising the Cauda Equina, providing protection for these structures while at the same time providing some flexibility and maneuverability to the individual. This flexible column is actually a stronger structure than a solid tubular structure, allowing this column to accommodate a variety of movements that the individual might undertake during the course of their lifetime. Obviously, such flexibility is also providing the individual with an adaptive advantage, recognizing the evolutionary contribution of the vertebrate spinal column. Furthermore, the spinal column provides a centering anchor for the physiologic chassis of the individual, thus supplying the individual with axial support.
Further adding to the biomechanical complexity of this structure, we noted that the human vertebral column is composed of 24 mobile and 9 fused vertebrae, with over 70 mobile joints and hundreds of attendant muscles and ligaments. It has become clear, as the knowledge of this biomechanical structure has expanded, that all of these components contribute to the proper biomechanical function of the spine. And, as a corollary, it is now being appreciated that abnormal function of any of these structures, alone or in combination, can result in back pain. Finally, there is a rich network of fine perforating nerves that encase the discs, epidural space, and facet joints. While many of these nerves are involved in the biofeedback processes that govern the spine and help maintain its ideal biomechanical adaptability, there is also a rich network of pain sensing fibers. These fibers, designed to inform the individual of derangements of the structures that contribute to the biomechanical patency of the spine, are obviously the final common pathways for the transmission of pain fibers. Understanding the fundamentals of how these nerves work, and why they ultimately transmit the sensation of back pain, is in its very infancy at this point.
While there is still a great deal to be learned in terms of the spinal biomechanics, there are certain fundamentals that are recognized by consensus as an acceptable foundation for this knowledge. At the heart of any discussion regarding spinal biomechanics are the essential concepts of “spinal stability” as well as “spinal instability.” While many volumes have been written about what ultimately comprises spinal stability, this still remains widely debated, even amongst experts.
However, most agree that the term “spinal instability,” reflects a condition in which the normal, physiologic movement of one or more segments of the spine has been replaced by a movement which is either excessive, or of an abnormal rotational or translational nature. The signature that excessive, translational, or rotational movement has become pathologic is either the production of (a) one or more neurological deficits associated with this abnormal movement; or (b) the appearance of reliably reproducible pain associated with that excessive movement.
In order to better comprehend these complex concepts, one useful paradigm that has been proposed is known as the “motion segment.” The motion segment by definition refers to any pair of adjacent mobile vertebrae of the spine, along with their intervening disc joint, associated facet joints, ligaments, tendons, muscles, and their exiting nerve roots and neural elements. Using this paradigm, one can attempt to first understand spinal biomechanics at a single level prior to attempting to understand the biomechanics of this articulated column as a whole. In order to do so, one must first appreciate that there is a range of normal movements that are necessary for the execution of the activities of daily existence.
In this range of normal movements, each motion segment is thought to contribute to the movements of the spinal column as a whole; in the first of these movements that we shall consider, when the individual leans forward, as though attempting to touch one's hands to one's toes, the spinal column assumes flexion. In flexion, presumably there is a foreshortening of the anterior aspect of the intervertebral disc space with somewhat of a distraction of the posterior elements of the motion segment—i.e. the spinous processes and the facet joints.
The converse of flexion is extension, in which the individual arches backwards as though to look skyward. Examining extension at the motion segment level, we see that the opposite of flexion now occurs—the spinous processes will somewhat approximate each other, and the anterior aspect of the disc will widen slightly. In addition, there is also lateral rotation and lateral bending. In lateral rotation, the torso is rotated around a theoretical axis directed along a craniocaudal axis through the mid-portion of the body. In lateral rotation, the shoulders and hips are rotated in such a fashion that the shoulder and hip on the side being rotated toward are drawn somewhat posteriorly, while the opposite shoulder and hip are drawn somewhat anteriorly; in true rotation, the ipsilateral shoulder is somewhat more posterior than the ipsilateral hip, while the contralateral shoulder is somewhat more anterior than the contralateral hip. In lateral bending, one will bring one arm towards the ipsilateral foot while elevating the opposite shoulder. By examining the body mechanics themselves, one can extrapolate the way that an individual motion segment would accommodate these movements.
Many detailed studies have been performed to identify the typical range of movement for these various normal physiologic movements. The problem, adding to the complexity of the biomechanical discussion, is that since human beings (and presumably their spines) come in many different sizes and shapes, attempting to postulate a “normal” range of movement can result in a number of inaccuracies. Rather, one must accept that to some degree, normal may often apply to normal for that individual, although there are certain basic standards that are going to be present throughout a broad range of the population.
With this as a foundation, we can now consider abnormal movements. For the purposes of this discussion, the abnormal movements under discussion would fall principally into two general categories:                1. An excessive amount of one of the natural movements that a motion segment is imbued with—i.e. flexion, extension, lateral rotation or lateral bending; and        2. An unnatural or non-physiologic movement between two vertebrae as they relate to one another, such as translation.        
It has already been previously stated that the amount of flexion, extension, lateral rotation, or lateral bending that is considered “excessive” varies substantially according to a patient's age, gender, and body habitus. It is ultimately, to some degree, a surgeon's judgment that determines “excessive” movement in one of these spheres of motion.
On the other hand, with respect to the second category, these movements are truly non-physiologic insofar that there are no mechanisms in the anatomy or physiology of the spine providing for such movements, nor are there any circumstances by which the spine would ever assume these movements.
The principle abnormal movement that is often under consideration is referred to, as translational movement. In general, the term “translational movement” refers to a pathologic condition in which one vertebra will transpose itself along either an anteroposterior axis or a lateral axis, while maintaining the same general orientation with respect to the craniocaudal axis. A typical example of translation is the condition known as spondylolisthesis. In a typical spondylolisthesis, the anterior aspect of one vertebra demonstrates an anticipated profile, as viewed in the lateral plane, but the entire vertebral body is shifted anteriorly, with respect to the next vertebra above or below. In other words, translation refers to an anteroposterior or lateral shift of one vertebra with respect to the other, while maintaining its primary craniocaudal orientation within the spinal axis. It is believed that subtle translational instability (perhaps even so subtle that good diagnostic evaluations to determine the presence of such a translational instability are not yet available to clinicians) may be responsible for a great deal of back pain associated with degenerative disease.
Yet another type of abnormal movement is rotational movement. In rotational movement, the motion segment demonstrates a situation in which one of the vertebrae will rotate around a hypothetical axis that is oriented in a craniocaudal direction and passes through the mid portion of the vertebral bodies of both vertebrae. The rotating vertebrae assumes movement which is similar to that which might be seen in lateral rotation, except in pathologic rotational movement, there is far excessive displacement of the vertebra with perhaps a relative disarticulation of one facet joint and relative obscuration of the other facet joints.
In addition to these movements, there are abnormal postures of the spine such as scoliosis, kyphosis, and excessive lordosis. These terms will be generally discussed below, but are not necessarily germane to a discussion functioning around the biomechanics of a motion segment.
Therefore, when examining this range of theoretic movements that a particular motion segment can undergo, one can then anticipate that nature would provide modulators of these movements, specifically designed to govern these movements and maintain them within the normal sphere of acceptable ranges of movement. The two chief governors of movements are the intervertebral disc joint, and the bilateral posterolateral facet joints. What is of particular importance is that these articulations do not exist in vacuum, exerting their effects on the movements of the spine in an isolated fashion as was once speculated; rather, these structures clearly work in a reciprocal fashion. While a large amount of attention has traditionally been given to the intervertebral disc joint, in recent years, the critical role of the facet joints has become increasingly recognized and appreciated.
In understanding the variable contribution that each of these lends to the physiologic range of motion, it appears in further analysis that in terms of flexion, or forward bending of the spine, the disc is probably the single most important modulator. Conversely, in extension, or backward bending of the spine, the facet joints appear to be particularly important. The role of the facet joints has been proven to be especially significant in biomechanical models and spinal lab models, where it has been extensively studied and shown that at a certain point of extension, the facet joints will essentially mechanically prevent any further extension. Lateral rotation and lateral bending appear to be a combination of input from the facet joints and the disc joint. In general, the current wisdom in spinal surgery contends that the progression of degenerative disease of the spine is characterized by laxity of associated ligaments, as well as loss of competency of the disc joints and facet joints resulting in excessive movement of the spine.
Yet another important consideration in studying the motion segment model is the neuroforamina, the canals through which the nerve roots exit the spinal canal. Theoretically, any condition incarcerating the exiting nerve roots can putatively lead to radicular symptoms. In recent years, it has become recognized that a number of pathologic processes can affect these foramina, including primary foraminal stenosis, as well as narrowing of the neuroforamina secondary to degenerative collapse of the disc. Every spine surgeon has encountered patients with virtually completely collapsed discs, particularly at L4-5 and L5-S1. What is now being recognized is that when this occurs, there is clearly a reduction in the size of the neuroforamina, which may be responsible for radicular symptoms in cases such as this. One surgical principle that remains somewhat controversial is the value of distraction of a collapsed disc with a relative restoration of the disc height. While this does remain controversial, from an anecdotal perspective, every spine surgeon has seen patients in whom elevation of the disc height and enlargement of the foramina does result in remarkable improvement of the symptoms.
Therefore, armed with at least this rudimentary understanding of spinal biomechanics, one can began to introduce rationale into the operative approaches that might be undertaken in patients with degenerative spinal disease.
Certainly, it at least appears logical and intuitive that when any anatomy is disordered, one basic principle of surgery would be to restore the anatomy to as “normal” as can possibly be achieved. This, of course, must be evaluated bearing in mind that in most instances, another principle of surgery is to remove as much pathologic tissue as possible. It is important to note that perhaps more in spinal surgery than in any other surgical subspecialty, the surgical treatment of advanced degenerative disease results in the removal of very extensive pathologically altered but “normal” anatomic tissues. With this type of procedure, there has been such extensive derangement that the normal anatomy can no longer be restored. Hence, one returns to the role that surgical fusion may play in attempting to provide the patient with at least some symptomatic results.
In a case such as that described, namely, extensive surgical removal of advanced pathologic disease, it is felt by many experts in the field that simply removing the pathologic tissue is an acceptable surgical procedure. This is typified by the multi-level laminectomy without fusion, the surgical approach that has been traditionally taken by many surgeons in the treatment of multi-level spinal steno sis, which is a time-honored operative approach to this problem that was considered satisfactory in many patients with advanced stenosis. However, that was partly because this population is often elderly and is not very physically active, either before or after the surgery.
But the demographics of spinal disease are clearly shifting. One important factor to consider is the changing profile of the aging population, here in the United States and around the world. In the United States, not only is the population aging, such that the median life expectancy has risen significantly over the past few decades, but also, we are seeing a population that is much more active more into their 7th and even 8th decades of life. This has had a clear effect on the philosophy governing surgeries for spinal stenosis. As mentioned previously, when the surgical approach to spinal stenosis was first being evaluated and undertaken, a multi-level laminectomy was the gold standard for surgical treatment, and it was clearly enough, as these patients were satisfied to live out their life in a relatively sedentary fashion.
However, in today's world, as mentioned above, there is a significant increase in the physical activities of the elderly population. This has resulted in a fundamental shift in the philosophy of this type of surgery. Clearly, doing a multi-level laminectomy in someone who is planning to continue to be very active is fraught with potential long term complications. Not only do these patients complain of a greater incidence of back pain after this type of surgery; more critically, a percentage of these patients develop translational and rotational abnormalities such as postoperative spondylolisthesis, kyphosis, or rotoscoliosis. With these potential complications, many surgeons now clearly favor an approach that incorporates fusion into the surgical philosophy.
However, the elderly population with spinal stenosis is clearly not the only cohort for which fusion is considered, and, in fact, represents the minority of patients. Again, with the shifting demographics including overall increase in the recreational physical activities of patients, high velocity motor vehicle accidents, and the ever increasing stresses of manual labor jobs, more and more young and middle aged patients are developing advanced disc disease, with a significant component of back pain. Returning to the original thesis put forth in this section, the current philosophy suggests that a major cause of back pain in that setting is microinstability and subtle translational movements that are seen in association with this advanced disc degeneration. Osteophytic spurring often seen on the anterior and posterior surfaces of these vertebral bodies is highly suggestive of this type of microinstability. In a patient with a clinical presentation that include back pain, with or without leg pain, in association with degenerative disc disease, loss of disc height, and at least inferential evidence of microinstability, it is now considered a very reasonable approach to move ahead with a spinal fusion, thus eliminating all movement from the motion segment in question. The pervasiveness of this surgical philosophy has lead to an almost logarithmic increase in spinal fusions over the past two decades, although more recently, these numbers have begun to stabilize. Currently, in the United States, an average of 300-400,000 spinal fusions is performed every year.
Again, it can be recalled that the theory of a spinal fusion is to eliminate excessive motion at a motion segment, simply by eliminating all motion. However, this then raises the question of the effect of this type of change on spinal dynamic might have on the intervertebral discs and facet joints at the next level above and/or below the fused disc. Bioengineering and biomechanics would dictate that fusing one level would have a relatively strong increase on the stress at the levels that are immediately adjacent to it and there have been some studies that have shown that there is an accelerated degeneration in discs that are adjacent to spinal fusions.
Obviously, there are also shortcomings with fusion, and many patients that have had a spinal fusion performed report chronic ongoing back pain well after otherwise recovering in an uneventful fashion. Of course, from both a bioengineering and intuitive perspective, obviously the most ideal approach that could be undertaken would be re-establishing the normal motion at the diseased or target motion segment. This would obviously satisfy even more of the biodynamic factors than a fusion, and would avoid the speculation that fusion leads to “adjacent disc disease.” Far more important, however, is the theoretical advantage of restoring/maintaining “normal” spinal dynamics, whatever this euphemism will ultimately be determined to imply.
It is interesting to note the correlation between the evolutions of the philosophies guiding the surgical treatment of arthritis affecting major joints and the philosophies guiding the treatment of spinal degenerative disease. The earliest surgical treatments of severe degenerative disease of the hip or knee joints typically involved fusion of these major joints. In that fashion, the pain from movement of the diseased joints was relieved by removing all motion from the joint. However, by the mid-1960's total hip joint replacement had been introduced, and total knee joints were introduced soon after. By the introduction of such devices it was thought that rather than losing the movement of these joints, returning the motion of the joint to a relatively normal range resulted in much better long-term results.
Pursuant to that philosophy, surgeons have been proposing various types of disc prosthesis for a number of years now. As far back as 1964, several Scandinavian surgeons attempted to address this problem by placing a steel ball or sphere in the center of a disc after removal had been completed. At least in the earliest part of this series (apparently, a total of 19 patients were treated in this fashion), no fixation device whatsoever was used, and the discs were merely distracted and the ball was placed into position. This experimental protocol as ultimately discontinued, but these surgeons recognized and demonstrated the need for an artificial disc of some kind. Of interest, there are at least two of these patients that remain with this sphere in place who are reported to still be doing very well.
In the 1980's, at the Charite' Hospital in Germany, a somewhat more sophisticated artificial disc was ultimately developed, and has been placed in several thousand patients in the 17 years since it was first introduced. This has recently been introduced in the United States as well, and has been cleared for clinical use on a non-trial basis. In this way, the theory is that even a badly diseased disc can be removed and hopefully, the natural movement of the lumbar spine can be recapitulated.
However, this Charite' disc is unfortunately, not without its issues and complications as well. Although the interpretation of the statistics is certainly subject to individual understanding, there is without question a number of patients who continue to do poorly, even after the placement of the Charite' disc. Nevertheless, this was essentially the first device introduced for clinical use, which attempted to speak to the issue of preserving spinal motion and returning it to normal. Since the introduction of the Charite' discs, a number of other devices designed to achieve “total disc replacement” have been brought to clinical trials, with many other devices essentially being proposed.
It is very clear, however, that spinal dynamics are of a sufficient level of complexity that these types of first and second generation devices are not going to be able to fully accommodate the demands placed on such theoretical device. Without question, further development is needed and the technology is probably years, if not a decade, away from a fully functional artificial disc that would truly satisfy all of the obligations and requirements placed upon it.
Spinal dynamics are clearly much more complicated than the dynamics at other joints, including the hip. There are multiple, complex reasons for this, but some of these reasons include the fact that any one (disc or facet) joint in the spine is, first of all, dependent on the actions of similar joints in the immediate areas (superior and inferior to the joint(s) in question), and probably, at least to a limited degree, to most of the other joints in the spine. Additionally, it is well-recognized that the dynamics that come to bear upon the spine are amongst the most complex biomechanical systems in the body.
One of the reasons why an artificial disc presents such a challenge to bioengineering is because of the many different and varying dynamic requirements and demands that are placed on the human discs. The load that it bears varies significantly depending on if the subject is lying, sitting or standing, as well as whether or not the subject is bearing any weight such as holding any additional weight. As a subject changes position (i.e. going from a crouched to a standing position) the internal pressure of the disc, as well as the dynamics of the motion segment will alter significantly. This is even further augmented by performing such a maneuver while bearing weight. The mechanical devices of the first and second generation artificial discs, which are currently on the market, do not have the wide range of responses to these many different mechanical stresses. Rather, these devices will respond to a very specific sequence of stressors, thus making them subject to mechanical fatigue over time. In addition, other problems may eventually present themselves including subsidence around the disc or alternatively possibly even autofusion. It should be noted that there have been several cases of autofusion of the cervical discs reported in the literature, and the mechanics and the biologic milieu for the theoretic basis of autofusion are very clear.
Additionally, there are probably no joints in the body that must move in as many planes and respond to shear stressors or load bearing in as many different planes as the intervertebral disc, with the lumbar intervertebral discs obviously subjective to the greatest pressures. In a word, the biomechanical demands on the intervertebral discs cannot be overstated. Yet another challenge to the artificial disc technology rests in the fact that the current state of knowledge regarding the pathophysiology and morphogenesis of degenerative disc disease remains in its infancy. As has been previously mentioned, there is data and evidence that is now coming to attention that suggests that degenerative disc disease is at least, in part, possibly related to a vasculopathy of the endplates. In that instance, it is not likely that an artificial disc is going to make much difference and degenerative processes will continue, in with the artificial disc in place. Over the long term, this could have significant, adverse effects on a level in which an artificial disc has been placed.
On the other hand, the theoretic basis of the use of an artificial disc, specifically the preservation of the natural motion of a motion segment, can be appreciated with great clarity. On the grandest hypothetical level, it certainly would be best to preserve motion in a structure that is as dynamic as the spinal column. We can only begin to speculate at the advantages of doing so, including the theoretic possibility of mechanically stressing levels above and below a fusion (as opposed to preserving the motion) as well as the possible salutatory effects on the spinal column as whole, and the issues regarding endplate vascular compromise.
It can be appreciated that the state of the art confirms what has already been hypothesized above: that as degenerative disease of the spine advances, normal movement at any level affected by this advancing disease is replaced by increased movement—indeed, ultimately pathologic movement.
One other issue that must be addressed is the fact that the actual surgical procedure for placement of an artificial disc, particularly in the lumbar spine, is a somewhat heroic effort. It requires an anterior transabdominal or retroperitoneal approach to the disc space in question, and particularly in the setting of an L4-5 operation, carries with it the potential risk of catastrophic or even lethal vascular injury to structures such as the aorta, the inferior vena cava, or the iliac vessels. Hence, the surgical, technical, and anesthetic challenges will sometimes disqualify a patient from the surgery who would otherwise greatly benefit from a motion preserving technology.
This creates a dilemma for many surgeons, such that many surgeons agree with the theoretic basis and indications for disc replacement, but are concerned with undertaking such a significant surgical procedure in order to place a device that may not truly accomplish the desired endpoints. Furthermore, in a number of cases, patients are not medically capable of tolerating this surgical procedure and its attendant anesthesia.
In an effort to identify technologies that offer motion preservation but are not attended by the technical challenges, it has been postulated that posterior motion preservation technologies may be an attractive alternative in certain instances. These systems are being euphemistically referred to as “dynamic stabilization” systems, although, in the end, analysis of such nomenclature demonstrates it to be oxymoron (dynamic=movement; stabilization=prevention of movement). However, semantics aside, what these systems are intended to do is to provide a system that utilizes the pedicles to anchor the device which then reduces excessive movements of a particular motion segment. Such a system might also provide an element of distraction, and in that way provide a partial “unloading” of the associated disc. The current level of wisdom suggests that both of these would have salutary effects.
The theoretic advantages of a posterior approach are well established. Firstly, and most importantly, this is a far easier technical approach than an anterior approach, and carries with it a substantially less chance of injury to a major vessel or a viscus; above all of that, a posterior approach often is necessary to treat the symptomatic lesion. Secondly, because it can be accomplished in a much shorter period of time, it requires far less anesthesia. Thirdly, the potential complications are significantly less, primarily due to some of the issues elucidated above. Fourthly, such devices as well as the surgical and hospital time for implanting them are far less costly than those associated with an artificial disc. This is obviously an advantage particularly in this day and age. Therefore, it has become apparent, that at least in some instances, it is desirable to achieve a surgical endpoint in which preserving the motion of the involved spinal segment is desired but performing the necessary surgery to accomplish “total disc replacement” is not appropriate.
In order to speak to this particular subset of patients, a number of devices have been recently introduced that can be placed through a posterior (traditional) approach and will preserve motion. Examples of this last strategy would include the posterior motion preserving device marketed by Scientix, as well as the Dynesis system.
The first posterior dynamic stabilization system that seems to have gained modest acceptance is the Scientix, a French company that has accomplished considerable notoriety throughout the spinal surgical world. This system involves a pedicle screw inserted into the superior and inferior vertebrae of a target motion segment, these screws being then connected to each other through a bridging component, which is the main focus of mobility in this device. This bridging piece, from a special design, allows for some flexion, extension, and rotation. The Dynesis System, marketed by Zimmer, offers an even simpler approach to the problem, which basically involves establishing pedicle screws fixation at the superior and inferior vertebra of the target motion segment, then hooking these together with a flexible material.
These systems are designed to be connected to the vertebrae of a target motion segment by pedicle screws. However, rather than the classic pedicle screw construct which involves connecting the pedicle screws to each other with a stabilizing, inflexible rod, these pedicle screws are connected to each other by a device that provides flexibility to the entire system. In the case of the Scientix device, the rod is designed to spin/rotate, as well as have some superior/inferior motion. In the case of the Dynesis system, the connecting device is made of a cord/fabric type material which is obviously flexible.
Yet other embodiments of dynamic stabilization systems have appeared. Recently, a device was introduced by Panjabi that provides a spring device to the ends of the connecting rods. Other devices will probably soon be introduced as well.
One issue that is often a focus of discussion is the role that the disc height contributes to the overall pathology. It is generally agreed upon that this is probably significant in a patient in whom the loss of disc height is associated with foraminal stenosis resulting in radiculopathy corresponding to the nerve root exiting at that level. However, the issues are much less clear in terms of discogenic pain syndromes. While many authors feel that the loss of disc height is associated with back pain, extensive review of the literature fails to demonstrate this to be a universal truth. Moreover, while theories regarding the genesis of such pain abound, none have been completely accepted. One interesting theory that has been alluded to above, and which has begun to gain momentum in recent times, is the possibility that disc height loss is related to ischemic changes in the end plates, which then lead to “discogenic pain syndrome.” Which comes first—the ischemic changes or the loss of disc height—is not yet known. But it is clear that in at least a subset of patients, these changes ultimately lead to further disc degeneration resulting in painful discopathy. What has been demonstrated clinically is that in properly selected patients, restoration of disc height with distraction of the associated vertebrae will result in improvement of radicular and/or back pain. Therefore, it seems logical that treatment approaches that are designed to preserve motion should also provide a mechanism by which disc height can be restored and maintained at the restored height level. There is clearly a need for such technology. The device herein disclosed differs from the previous art in several important aspects. Firstly, it can be inserted using a minimally invasive approach. Secondly, the herein disclosed is unique insofar that this device can be adjusted in terms of the amount of flexibility that it provides to the target motion segment. Thirdly, this device can be inserted into the target motion segment or segments and then distraction can be applied, by which disc height can be restored and maintained. Finally, this device can be converted to a classic non-dynamic system in order to encourage the establishment of a fusion in its place. Such a device, and system of implantation, is unique, useful, novel, and non-obvious.